1. Field of the Invention
The present invention relates to an original size detecting apparatus for detecting a size (a length) of an original placed on a document platen in an image forming apparatus such as a copying machine or an image reader.
2. Description of the Related Art
A conventional image forming apparatus such as a copying machine or an image reader is provided with an original size detecting apparatus for automatically detecting a size of an original placed on a document platen. Copy paper of a suitable size is selected according to the detected original size or the lens is moved according to the original size and a copying magnification.
The conventional original size detecting apparatus comprises a photosensor for receiving light reflected from an original and outputting a signal according to the intensity of the light, a comparator for comparing the output signal of the photosensor with a specified reference value independent of the original size and outputting a signal indicating presence or absence of the original, whereby the original size is determined when the signal is outputted from the comparator (as indicated in Japanese Patent Publication No. 4974/1985).
However, there are various densities and patterns of originals and intensity of light from an original varies considerably dependent on the density or pattern. For this reason, it is very difficult to set a reference value for determining presence or absence of various originals. In addition, it may occur that the intensity of the reflected light exceeds the reference value in spite of the presence of the original to output a signal indicating absence of an original, and thus it is difficult to detect presence or absence of an original reliably.
Consequently, in the above described conventional original size detecting apparatus, it may happen that the length of an original is determined to be shorter than the actual length, that is, the original size is determined to be smaller than the actual size.